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The Charles River Pollution Control District (CRPCD) in Medway, Mass., identified a need for improved efficiency, then found a way to fill the gap and pay for a project that ended up costing $850,000.

The 30-year-old 5.7 mgd regional wastewater treatment facility had a major upgrade in 2000 when it added an anoxic selector, fine-bubble aeration, a SCADA system, and cloth disc filters. The latest improvements came after an energy audit funded by the Massachusetts Division of Energy Resources in 2008.

“They approached us to see if we were interested in participating in the Massachusetts Energy Management Pilot for Drinking Water and Wastewater Facilities,” says executive director Cheri Cousens. The program has a goal of helping plants reduce energy use by 20 percent, and the Charles River project will meet or exceed that goal.

Two turbo blowers replaced three older centrifugal blowers to provide dissolved oxygen to the activated sludge process more efficiently. “We’ll save on energy costs just by using a different piece of equipment,” says Cousens.

A 20 kW solar photovoltaic system was added to the roof of the building that houses the blowers. While it’s a small solar project, it will help meet the plant’s energy needs as it serves the treatment needs of Medway and three neighboring communities, while also handling septage from seven other towns.

Assessing the need

Charles River’s plant was one of seven that took part in the state energy efficiency pilot program. The work started with an energy audit that found the plant spent about $454,000 a year for energy, using 2,000 Btu per gallon per day of wastewater treated. That was about 1,000 Btu below average, but still allowed for improvement.

The study found that the plant site was also a good candidate for solar energy, but not for wind or inline hydroturbine in the outfall. The original study recommended a 120 kW photovoltaic system, but a later analysis found that the 20 kW installation would be less expensive and would free up money for the higher efficiency turbo blowers.

“Our engineering company, CDM, recommended the smaller solar unit because we would save more power with the turbo blowers than would be produced by the larger solar project, while investing the same amount of money,” says Cousens. “It’s better not to need the power in the first place.”

In fact, the approach lowered the total cost of the equipment and installation from the estimated $1 million to the final cost of $850,000. The district used State Revolving Fund loans to pay for the work, then received 100 percent funding to repay the loans through the federal American Recovery and Reinvestment Act stimulus program.

Results

The solar unit went into operation in February 2011, and the turbo blowers came online in August. In the long run, the project is expected to reduce the plant’s energy bills by more than $100,000, a 22 percent savings. That exceeds the state program’s goal. With the smaller solar project and the turbo blowers, it also exceeded the original estimate of a 16 percent energy savings. The efficiencies will also reduce carbon dioxide emissions by 558 tons per year.

Cousens says further savings will come with the future addition of variable-speed drives to the plant’s water system. In addition, the district has already saved by bidding its electricity on the open market. Rather than paying 7.5 to 8 cents per kWh from the local utility, it is paying 6.74 cents from Hess Corporation, an east coast energy supplier.

Understanding and analyzing its energy needs helped the district improve its energy efficiency, minimize costs for customers, and reduce the treatment plant’s impact on the environment.